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May 10, 2007 -- It's still a mystery why women suffer more knee injuries than men do. And an elaborate new study fails to link ACL rupture to menstrual cycle.

The ACL -- the anterior cruciate ligament -- helps keep the knee stable. When overloaded, the ACL ruptures. This is extremely painful, as sport fans know from the familiar sight of athletes collapsing in agony. It also takes a long time to heal.

Female athletes suffer ACL rupture much more often than do male athletes. In soccer and basketball, women suffer ACL injuries three times more often than men do. While running obstacle courses during military training, female recruits suffer 11 times more ACL injuries than do male recruits, note the researchers.

Why? There's some evidence that ACL injuries are more common during certain phases of the menstrual cycle, although studies differ as to exactly which phase is to blame.

To settle the question, Ohio State University researcher Ajit M.W. Chaudhari, PhD, and colleagues used video tapes to analyze men and women as they performed various exercises that stressed their knees -- their ACLs in particular.

The study enrolled 12 men and 25 women. Thirteen of the women were taking oral contraceptives. Each woman underwent testing at different phases of her menstrual cycle.

The result: Regardless of their menstrual phase or contraceptive use, women put no more strain on their ACLs or related muscles than did men.

"Hormone cycling in women does not appear to affect either knee joint or hip joint loading," Chaudhari and colleagues report. "Women do not appear to have significantly different loads than men during any phase of the menstrual cycle. Moreover, the use of an oral contraceptive does not appear to affect joint loading."

If there is an effect of sex hormones on knee injury, the researchers suggest, it must be part of a complex interaction between hormones, ligament structure, fatigue, and "neuromuscular control."

The study appears in the May issue of The American Journal of Sports Medicine.